CALICE Digital Hadron Calorimeter: Calibration and Response to Pions and Positrons

1. CALICE Digital Hadron Calorimeter: Calibration and Response to Pions and Positrons Burak Bilki Argonne National Laboratory University of Iowa (S)DHCAL Meeting January 15, 2014 Lyon, France

2. Digital Hadron Calorimeter (DHCAL) • Each layer with an area of ~ 1 x 1 m2 is read out by 96 x 96 pads. • The DHCAL prototype has up to 54 layers including the tail catcher (TCMT) ~ 0.5 M readout channels (world record in calorimetry!) • Concept of the DHCAL • Imaging hadron calorimeter optimized for use with PFA • 1-bit (digital) readout • 1 x 1 cm2 pads read out individually • (embedded into calorimeter!) • Resistive Plate Chambers (RPCs) as active elements, between steel/tungsten

3. DHCAL Data Muon Trigger: 2 x (1 m x 1 m scintillator) Secondary Beam Trigger: 2 x (20 cm x 20 cm scintillator) y z x Nearest neighbor clustering in each layer Event Particle ID: Čerenkov/μ tagger bits Hit:x, y, z, time stamp Cluster: x, y, z

4. Calibration/Performance Parameters Efficiency (ε) and pad multiplicity (μ) • Track Fits: • specifically for muon calibration runs • Identify a muon track that traverse the stack with no identified interaction • Measure all layers • Track Segment Fits: • for online calibration • Identify a track segment of four layers with aligned clusters within 3 cm • Measure only one layer (if possible) • Fit to the parametric line: x=x0+axt; y=y0+ayt; z=t A cluster is found in the measurement layer within 2 cm of the fit point? Yes No ε=1 μ=size of the found cluster ε=0 No μ measurement muon 8 GeV secondary beam

5. Calibration of the DHCAL RPC performance Efficiency to detect MIP ε~ 96% Average pad multiplicity μ~ 1.6 Calibration factors C = εμ Equalize response to MIPS (muons) Calibration for secondary beam (p, π±,e±) If more than 1 particle contributes to signal of a given pad → Pad will fire, even if efficiency is low → Full calibration will overcorrect Simulation Full calibration Hi… Number of hits in RPCi More sophisticated schemes J. Repond - The DHCAL

6. Calibration Procedures 1. Full Calibration 2. Density-weighted Calibration: Developed due to the fact that a pad will fire if it gets contribution from multiple traversing particles regardless of the efficiency of this RPC. Hence, the full calibration will overcorrect. Classifies hits in density bins (number of neighbors in a 3 x 3 array). 3. Hybrid Calibration: Density bins 0 and 1 receive full calibration. Hi… Number of hits in RPCi

7. Density-weighted Calibration Overview Warning: This is rather COMPLICATED Derived entirely based on Monte Carlo Assumes correlation between Density of hits ↔ Number of particles contributing to signal of a pad Mimics different operating conditions with Different thresholds T Utilizes the fact that hits generated with the Same GEANT4 file, but different operating conditions can be correlated Defines density bin for each hit in a 3 x 3 array Bin 0 – 0 neighbors, bin 1 – 1 neighbor …. Bin 8 – 8 neighbors Weighs each hit To restore desired density distribution of hits

8. Density-weighted Calibration Example: 10 GeV pions: Correction from T=400→ T=800 Mean response and the resolution reproduced. Similar results for all energies.

9. Density-weighted Calibration Expansion: To large range of performance parameters • GEANT4 files • Positrons: 2, 4, 10, 16, 20, 25, 40, 80 GeV • Pions: 2, 4, 8, 10, 10, 25, 40, 80 GeV • Digitization with RPC_sim • Thresholds of 200, 400, 600, 800, 1000 (~ x 1fC) • Calculate correction factors (C) • for each density bin separately • as a function of εi, μi, ε0 and μ0 (i : RPC index) • Plot C as a function of R = (εiμi)/(ε0μ0) • → Some scattering of the points, need a more sophisticated description of dependence of calibration factor on detector performance parameters π: Density bin 3 25 GeV R = (εiμi)/(ε0μ0) Only very weak dependence on energy:Common calibration factors for all energies!

10. Density-weighted Calibration: Empirical Function of εi, μi, ε0, μ0 25 GeV Positrons Pions Calibration factors π: Density bin 3 π: Density bin 3 Functional dependence of calibration factors depends on particle type! (Due to different shower topologies: Different density distribution leads to different impact of performance) R = (εiμi)/(ε0μ0) Calibration factors R = (εi0.3μi1.5)/(ε00.3μ01.5)

11. Density-weighted Calibration: Fits of Correction Factors as a Function of R Power law Fit results for p=pion, similar results for p=positron Correction Factors Rπ+ Rπ+ Rπ+ Correction Factors Rπ+ Rπ+ Rπ+ Correction Factors Rπ+ Rπ+ Rπ+

12. Calibrating Different Runs at Same Energy 8 GeV e+ 4 GeV π+ Uncalibrated response (No offset in y) Full calibration (+5 offset in y) Density – weighted calibration (-5 offset in y) Hybrid calibration (-10 offset in y)

13. Comparison of Different Calibration Schemes χ2/ndfof constant fits to the means for different runs at same energy → All three schemes reduce the spread of data points: Benefits of calibration! No obvious “winner”: Similar performance of different techniques

14. Linearity of Pion Response: Fit to N=aEm Uncalibrated response 4% saturation Full calibration Perfectly linear up to 60 GeV (in contradiction to MC predictions) Density- weighted calibration/Hybrid calibration 1 – 2% saturation (in agreement with predictions)

15. Backup

16. Particle Identification (PID) 0. Čerenkov counter based PID (good for 6, 8, 10, 12, 16, and 20 GeV) 1. Topological PID: Starts with the trajectory fit (used for 2, 4, 25 and 32 GeV) Topological Variables 16 GeV • Visually inspect the positron events in the ~10% excess in the topological PID • They are positrons • 10 % compatible with the inefficiency of the Čerenkov counter, which was ≥ 90% for most energies • Topological PID works nicely!